The present invention relates in general to flaps such, for example, as control flaps of the type which are conventionally hingedly connected to the trailing edge of a stationary foil on a vessel--e.g., a hydrofoil vessel or the like--and, more particularly, to an improved flap construction which permits optimization of the discontinuity or gap required between the flap leading edge and the foil trailing edge at all points along the entire span of the foil irrespective of the fact that in those specific areas where the flap is hingedly connected to the foil, a "hard" connection is formed and, consequently, little, if any, differential deflection occurs; yet, wherein those regions of the flap intermediate adjacent points of "hard" hinged connection--i.e., those regions intermediate "hard" points spaced apart in a span-wise direction--permit of maximized differential deflection between the two edges resulting from the pressure of the fluid medium through which the flap/foil combination is moving without any possibility of interference between the flap and the foil.
Although the present invention finds particularly advantageous use in connection with flap/foil combinations as used on hydrofoil vessels and the like and will, therefore, be described in such an environment, those skilled in the art will readily appreciate as the ensuing description proceeds that the invention is not so limited and may find advantageous application in other environments such, for example, as control flaps on the trailing edges of airfoils used with aircraft.
It has long been recognized that trailing edge flaps for vessel control--particularly, for control of hydrofoil ships--are desirable and, indeed, often essential in order to permit reliable controlled maneuverability of the vessel; and, consequently, it has been a common practice to hingedly connect a plurality of such trailing edge flaps to the trailing edge of a foil with the leading edge of the flaps spaced from, but in close proximity to, the trailing edge of the foil. In such arrangements, it is also generally common for adjacent flaps to be interconnected in end-to-end fashion with each flap serving to drive at least one adjacent flap and with each flap (except for that flap or those flaps directly connected to external drive mechanisms) being driven by an adjacent flap. However, regardless of whether any given flap is functioning as a drive flap, a driven flap, or both, its leading edge is generally hingedly connected to the foil trailing edge at two points spaced apart in a span-wise direction; and, at those points of hinged connection, the flap leading edge and foil trailing edge will generally be deflected in like amounts and in unison by the pressure of the fluid through which the vessel is moving. Consequently, in these localized spaced regions of "hard" connection, it is theoretically possible and relatively simple to design mating edge contours which ensure maintenance of optimized gaps or discontinuities therebetween at all times and under all operating conditions.
Unfortunately, however, in those regions of the mating flap leading edge and foil trailing edge which are located between the span-wise spaced hinged points of "hard" connection, dynamic conditions are such that in operation the pressure of the fluid medium through which the vessel is passing serves to cause significant deflection of both the foil (and its trailing edge) and the flap (and its leading edge). While the degree of deflection between such foil trailing edge and flap leading edge is substantially the same at the spaced "hard" points of hinged connection therebetween, in the "soft" regions intermediate such spaced "hard" points the degree of deflection between the two edges can be, and often is, significantly different.
Indeed, when dealing with a foil having a free tip that is not directly and positively connected to the vessel structure, the relative deflections of the flap leading edge and the foil trailing edge can be in opposite directions. For example, assuming that the vessel is a hydrofoil ship moving through water and that the trailing edge control flap(s) is(are) shifted through a downward or negative angle of rotation for the purpose of improving lift and/or controlling maneuverability, those skilled in the art will appreciate that fluid pressure applied to the bottom surfaces of the flap/foil combination will cause the outer tip of the foil to be deflected upwardly to a greater extent than inboard regions thereof, thus producing a foil trailing edge contour that is slightly concave rather than linear. Considering any given flap having its leading edge hingedly connected to such concave foil trailing edge at two spaced span-wise points, it will be appreciated that at the two points of "hard" or hinged connection, the flap leading edge will move with the foil trailing edge and, hence, at those two points the gap or discontinuity therebetween remains substantially constant. But, intermediate those two points, fluid pressure exerted by the water through which the vessel is moving will be applied directly to the under-surface of that flap, causing the flap and its leading edge to be deflected upwardly in the intermediate unconstrained region of the flap. That is, the central portion of the flap leading edge will be cambered or bowed upwardly so that the flap leading edge assumes a somewhat convex shape in the region of the concave foil trailing edge, thus causing interference between the two edges with resultant reduction in fatigue life thereof and/or significant decrease in the size of the gap or discontinuity between the flap leading edge and the foil trailing edge along the upper surface of the flap/foil combination; while, at the same time, significantly increasing the size of the gap or discontinuity between the two edges along the lower surface of the flap/foil combination. A somewhat similar result occurs even when the two edges are deflected in the same direction since the two edges tend to be deflected by different amounts, particularly at the mid-point of the flap leading edge.
The foregoing differential deflection problems have, for a long period of time, presented severe design problems for foil designers. Thus, while control flaps are desirable and, indeed, essential in many applications, the need to provide a gap or discontinuity between the flap and the foil at the juncture thereof tends to increase drag and to lessen the effectivity of the flap. The degree of degradation in flap performance has been found to be directly related to the size of the discontinuity or gap between the foil trailing edge and the flap leading edge.
Prior to the advent of the present invention, various attempts have been made to solve the problems introduced by varying discontinuities at the junction of the flap leading edge and the foil trailing edge. One such attempt has involved the use of adjustable flap hinges; an approach involving cumbersome and expensive assembly procedures requiring the use of separate shims. Unfortunately, during routine periodic maintenance there is a distinct possibility that one or more of such shims will be removed and will not be replaced or, if replaced, will be improperly positioned, thereby promoting flap/foil interference, reducing fatigue life, increasing drag, and decreasing flap effectivity. Moreover, fatigue life of the foil is further severely reduced because such adjustable hinge arrangements introduce undesired stress concentrations at localized points.
A second approach that has been employed, but which has been found to be entirely unsatisfactory, has been that of simply providing a sufficiently large gap or discontinuity at the hinged connections and, therefore, along the juncture of the flap leading edge and foil trailing edge in the span-wise space intermediate the hinged connections, that flap/foil interference is precluded even under those operating conditions when the edges are subjected to maximum differential deflection. Although this approach has eliminated the problems of reduced fatigue life, cost, and difficulties in assembly procedures, at the same time the excessively large discontinuities or gaps have further increased drag and reduced flap effectivity.
Other prior art of purely general interest to the subject of flap/foil design problems include that disclosed in, for example, U.S. Pat. No. 3,044,432 Wennagel et al.; U.S. Pat. No. Re. 26,059 Meyer et al.; U.S. Pat. No. 3,347,197 Scherer; and, U.S. Pat. No. 3,934,533 Wainwright.